Corresponding A/D converters, in spite of their relatively inferior accuracy, and relatively slower components, should have a high signal/noise-ratio, and are producible according to conventional technology. Possible applications are, for example, codec filters for telephone purposes and coding filters for the digitizing of audio signals for high quality digital pick up. Digitizing is understood to mean that a number code is attached to the scanning value of an analog waveform, and is proportional to the size thereof.
A/D converters may work according to different methods, and in doing so, the commonly used methods as a rule require the use of very precise working components. In order to reduce the required precision for the analog components, which are required for the conversion process, the delta modulation method has recently been used. The delta modulation techniques, however, require very high scanning rates. As illustrated in the publication by K. Niwa, A. Yukawa, A. Tomozowa, "A Discretely Adaptive Delta Modulation Codec", IEEE Transaction on Communications, Vol. Com-29, No. 2, Feb. 1981, pages 168 to 173, it is seen that the delta modulation codec has a so-called slope adaptive circuit in the feedback circuit, which causes large gradients of the input signal to correspond to large level variations of the quantization output (and vice versa). However, the codec requires a very high frequency for the operation of the slope adaptive circuit. Furthermore, because the scan frequency is high, the adaptation is quasicontinuous and can, therefore, only respond relatively slowly to a step-shaped input impulse. Eventually, this codec has an analog integrator and merely a comparator on the input side.
As illustrated in the publication by T. Last "Proportional Step Size Tracking Analog to Digital Converter", Rev. Sci. Instrum., 51 (3), Mar. 1980, pages 369 to 374, an A/D converter is known, which likewise has a slope adaptive circuit. This circuit is formed from a quantizer and an upward/downward counter. This circuit, however, has no analog circuit nor analog function, and acts as a simple A/D converter and not as a delta modulator. Furthermore, this converter likewise carries out no integration on the input side, and therefore has a high quantizer noise level. There is no feedback of errors in the modulation cycle, in order to reduce the accumulated error in the following cycle.
As illustrated in the publication by F. de Jager, "Delta Modulation a Method of PCM Transmission using the 1- Unit Code", Phillips Res. Rept., Vol. 7, pages 442 to 466, 1952, delta modulation is known, which uses a slope adaptive modulation circuit in the form of an analog integrator. This system has two analog integrators, one of which is disposed in the input circuit. The utilization of two analog integrators leads to a considerable reduction of the tolerance of the analog components and causes considerable design problems for the creation of a stable circuit, and is economically unfavorable.
As illustrated in the publication by J.C. Candy, "A use of Limit Cycle Oscillations to Obtain Robust Analog to Digital Converters", IEEE Trans. on Communications, Vol. COM-22, Bo. 3, Mar. 1974, pages 298 to 305 and B. A. Wooley, I. L. Henry, "An Integrated Per-Channel PCM Encoder Based on Interpolation", IEEE Journal of Solid-State Circuits, Vol. SC-14, No. 1, Feb. 1979, pages 14 to 20, are known A/D converters, in which through the disposition of an integration and amplification element in the input circuit, the scanning rate is reduced and the signal/noise ratio is increased. However, the feedback is formed of a direct feedback of the quantization of the output of the integration and amplification element, so that the number of possible digital estimates of the input signal are reduced to the number of the quantization level of the quantizer. Furthermore the circuit is not slope adaptive.
It is accordingly an object of the invention to provide a method and apparatus for A/D conversion, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type, which works according to the delta modulation principle, which supplies a high signal/noise-ratio with relatively small scanning rates and with high precision, which tolerates considerable deviations in the value of its components without large errors in the digital output signal and without instability of the circuit, and consequently has a large stability range.